Boiling Point Lab

In class, we did a lab to determine the boiling point of rubbing alcohol. We used a thiele tube filled with water and place a test tube attached to a thermometer w/rubber band in it. However, we never saw any signs of boiling. We conducted several trials, the temperature went all the way above 93 degrees, but there was no sign of boiling.

Our teacher said we overshot it..but I don't think that makes sense. Even if the temperature of the water exceeded the boiling point of the alcohol, the alcohol should have begun to boil.

Also, I don't understand the point of using the water and thiele tube setup. Why not directly heat the rubbing alcohol and measure the temperature in it? The temperature will stop changing at the boiling point allowing for easy detection.

Staff: Mentor

No we did not. Do you know why we didn't see it boil? And why not directly heat the rubbing alcohol and measure the temperature in it? The temperature will stop changing at the boiling point allowing for easy detection.

Staff: Mentor

Capillary tube plays exactly the same role boiling stone does.

Your teacher explanation as quoted ("you overshot") doesn't sound correct. Yes, it is possible the liquid was overheated, other than that it is not possible to miss the boiling point. Your thinking (boiling should keep the temperature "locked" at the boiling point) is right.

It might take a very small unit of time, but while it is disappearing you can still read the temperature....correct?

So I read that superheating occurs when surface tension added with atmospheric pressure creates the need for greater vapor pressure. But when and why does this happen? And why in particular must it have happened in this particular lab?

Whenever there are no places on which the bubbles can start to build. So called "nucleation sites". If the liquid is pure enough, if the walls of the container are smooth enough, vapor bubbles don't build up easily enough.